In Real Time Locating Systems, which utilize a Time Difference Of Arrival of messages transmitted by location tags and received by location receivers, time base synchronization between location receivers is a key task.
In the context of the present specification, synchronization is to be understood as a determination of a synchronization correction value, which describes the difference between the time assessed in accordance with the different time bases of the location receivers at a given moment. Whether this information is subsequently used to actually adjust the time bases or to correct TDOA values which were generated using these time bases is irrelevant. The latter use of synchronization is sometimes called “virtual synchronization”, for instance in U.S. Pat. No. 6,968,194 B2.
In many products synchronization is achieved by connecting the location receivers, which herein will also referred to as location anchors, with a reference time base over a wired connection. Obviously, however, a wired connection is expensive to install and in some cases may even be impossible to install.
Several methods have been developed in order to perform said synchronization via a wireless channel. Such methods use a reference device, which transmits a reference signal. For example, US 2004/0108954 A and U.S. Pat. No. 7,492,316 B1 describe the use of a “reference tag”, which transmits a sequence of pulses as a reference signal, for distributing reference timing within a network of location receivers.
Another system described in U.S. Pat. No. 7,528,776 B2 makes use of the fact that the reference device is positioned at a known location.
Another prior-art Real Time Locating System that uses a wireless channel for synchronization is shown in FIG. 1. It comprises location receivers 100, reference devices 101, location tags 102 and a location server 104. The location receivers 100 are located at known positions. The reference devices 101, which will also be called reference tags herein, are also located at known positions. The location tags 102 are those devices, for which the current position shall be identified by the RTLS.
For synchronization purposes, the reference tags 101 periodically broadcast messages, which are received by the location receivers 100. Based on the respective Times Of Arrival (TOAs), time differences of arrival (TDOAs) between the respective times of reception of these messages by different location receivers 100 are calculated. Given the known positions of the reference tags, the synchronization between the location receivers 100 is calculated.
In order to determine the location of the location tags 102, the location tags 102 broadcast messages, which are received by at least some of the location receivers 100. The respective TOAs of these messages are recorded at the location receivers 100 and reported to the location server 104. The location server 104 calculates the TDOAs between location receivers. Based on the TDOAs and the known positions of the location receivers 100 the location server 104 determines the location of the location tags 102.
In practice, a message which is transmitted by the reference tag 101 and received by location receivers 100 is subject to multipath propagation. This multipath propagation causes an error in the synchronization of the location receivers 100. Thus, the synchronization between location receivers 100 and the subsequent calculations of TDOAs and locations of location tags may be highly inaccurate in a multipath environment.
US 2008/0095131 A1 addresses the problem of multipath propagation of reference messages by performing a distance measurement between a reference tag, (which is called “sync unit”), and the location receivers. Basically, two-way ranging measurements between location receivers and reference tags are suggested. Two way ranging has been described for example in annex D1.3.1 of IEEE 802.15.4a-2007. For this purpose, the location receivers are also capable of transmitting messages. Hence the location receivers of this system might actually be called location transceivers. A drawback of the approach disclosed in US 2008/0095131 A1 lies in the fact that the accuracy of the two-way ranging will strongly depend on the accuracy of the time bases of the location receivers involved. Furthermore, this approach puts additional hardware complexity and additional processing load on the location receivers by requiring the handling of the two-way ranging process, which consumes additional airtime and energy.